1. Field of the Invention
The present invention relates to a radar apparatus.
2. Description of Related Art
There is known a radar apparatus configured to transmit a laser light, receive the laser light reflected from a reflecting object, and determines a distance to the reflecting object on the basis of a time between a moment of transmission of the laser light and a moment of reception of the laser light. For example, refer to Japanese Patent Application Laid-open No. 9-236661.
In order to accurately determine a reception time at which the laser light is received, such a radar apparatus includes a signal processing circuit operating to detect a time width during which a reflection signal having a signal level depending on the received laser light exceeds a predetermined threshold, and compensate for an error in the reception time depending on the intensity of the received laser light. The reason why such a compensation is performed is that the reflection signal originating from the received laser light has characteristics that it rises sharply, and falls slowly, and that the rising edge slope and the falling edge slope varies depending on the intensity of the received laser light. If a time at which the reflection signal exceeds the predetermined threshold is simply determined as the reception time of the reflected laser light, the reception time thus determined varies depending on the intensity of the received laser light.
Accordingly, in the conventional radar apparatus as described above, a mid-time between a time at which the reflection signal exceeds the predetermined threshold and a time at which the reflection signal falls below the predetermined threshold is calculated, and a correction time by which the calculated mid-time is corrected is calculated on the basis of the intensity of the received light. More specifically, the correction time is determined by referring to a graph showing a correspondence relation between the correction time and the time width during which the reflection signal exceeds the predetermined threshold. This correspondence relation is obtained in advance through experiment or simulation. The mid-time is subtracted by the correction time, and then determined as the reception time of the reflected laser light at which the reflection signal is largely at its peak.
It is also known as disclosed, for example, in Japanese Patent Application Laid-open No. 2004-177350 to improve the detection sensitivity of a radar apparatus of the type described above, in which a laser light is emitted by a predetermined number of times repeatedly for a given emission area, and a plurality of light reception signals each containing a reflection signal resulting from this multiple emission of the laser light are integrated in order to increase the intensity of the reflection signal corresponding to the laser light reflected from a reflecting object.
In such a radar apparatus, if a laser light reflected from a reflecting object having a very large intensity is received, which may occur in a case where the light reflecting object is in the close vicinity of the radar apparatus, or the light reflecting object has a very high reflectivity for the laser light, the signal level of the reflection signal produced corresponding to the received laser light may have a signal level beyond the level which signal processing circuits included in the radar apparatus can deal with. As a result, the reflection signal is saturated, and its waveform is deformed.
Especially, in the case where a plurality of light reception signals are integrated to improve the detection sensitivity, the above problem of the signal saturation and the waveform deformation can easily occur.
If there occurs the saturation and deformation of the reflection signal, it becomes difficult to accurately determine the reception time of the laser light even when the correction as disclosed in Patent Application Laid-open No. 9-236661 is performed.
Incidentally, the radar apparatus disclosed in Japanese Patent Application Laid-open No. 2004-177350 in which the light reception signals containing the reflection signals are integrated to improve the detection sensitivity is configured to eliminate background noise from the integrated signal in order to further improve the detection sensitivity. In more detail, in this radar apparatus, during a period in which a polygon mirror having a plurality of reflecting surfaces for reflecting a laser light generated by a laser diode is in a state where a switching of the reflecting surfaces is on its way, a laser light is intentionally emitted toward outside a target emission area, and background noise is calculated on the basis of a light reception signal obtained at the time. This makes it possible to calculate background noise containing periodic noise components due to a clock pulse of a CPU included in the radar apparatus, and electromagnetic noise due to generation of the laser light. By eliminating the background noise from the integrated signal, it becomes possible to increase S/N to thereby further improve the detection sensitivity of the radar apparatus.
The background noise needs to be calculated in a specific timing as explained above, and stored in a memory such as RAM each time it is calculated, because the background noise varies with time due to secular change of the characteristics of the radar apparatus, and change of thermal environment. Accordingly, the conventional radar apparatus of the type as disclosed in Japanese Patent Application Laid-open No. 2004-177350 has a problem in that the load of the CPU becomes very large, because the CPU has to calculate the background periodically.